Field of the Invention
The invention relates to a field device having at least one switching converter circuit and a measuring circuit for determining at least one measuring variable, wherein the switching converter circuit is supplied with electric power on the input side and the switching converter circuit is connected to the measuring circuit on the output side and the switching converter circuit supplies the measuring circuit with electric power.
Description of Related Art
Field devices of the type mentioned above have been known for a long time in process automation and are used for monitoring processes and media handled in the processes due to their ability to determine a measuring variable or also several measuring variables. Such field devices are, for example, configured for determining the flow of a flowable medium through pipes or channels, in that, for example, measuring devices are used that operate using the Coriolis or vortex principles, in which ultrasonic signals are used or which determine the flow using calorimetry. Other field devices of this type can be designed for determining the fill level of a medium, wherein the medium can be, for example, a liquid or also a bulk material. Thereby, it is known from the prior art to evaluate the transit time of electromagnetic signals according to the radar principle or, more specifically, the TDR principle. The Doppler effect is also applicable or plumb lines are used. Further, there is a plurality of field devices for recognizing when a fill level is reached, either capacitively, inductively or using mechanical, swingable elements. Other field devices equipped with a measuring circuit are used for recording a pH value, the temperature, the viscosity or an electrical conductivity of the medium of interest in the process. The details of the measuring variable recorded or determined with the measuring circuit of the field device are not the main point here, rather aspects of the invention described in the following can be used for all field devices having a switching converter circuit and having a measuring circuit.
The switching converter circuit provided in the field device is used essentially for energy supply of the measuring circuit, wherein the switching converter circuit is employed for supplying a defined voltage—usually direct voltage—on the output side, which the measuring circuit needs in order to properly function. Measuring circuits are often used not only for detecting raw measuring variables, but also for processing, filtering, adapting, and converting of signals.
Switching converters, which are also often called switching regulators, are based on the principle of at least one switched mode power supply. Thus, it is possible to boost an input-side direct voltage, for example, using the clocked interruption of the current path with a coil as power supply and subsequent smoothing of the generated voltage peaks. Very different types of switching converters are known, including buck converters, the above-described boost converters, and inverting buck-boost converters, which can supply higher as well as lower (inverted) voltages at the output-side than the voltage that was supplied to the switching converter at the input side.
Switching converter circuits are usually no longer discretely constructed in modern field devices, rather, semiconductor devices provided especially for implementing switching converter circuits are used, which, depending on the specific application, are equipped with an external circuit. The external circuit mostly comprises the always-necessary power supply—usually in the form of a coil—and other circuit parts such as feedback networks and filter circuits. Depending on the input voltages and currents on the output-side load of the switching converter circuit and the corresponding desired output voltages and currents, these circuit parts are often dimensioned according to specifications of the manufacturer of semiconductor parts in order to operate the switching converter circuit overall at the best possible operating point for the specific use.
Due to the switched mode, the switching converter circuit emits interfering signals that, for example, influence the measuring circuit connected at the output side in an undesired manner. The occurrence of different types of interferences and of interference changing over time is particularly problematic since it is difficult to respond to them in terms of circuitry.